1. Field of the Invention
The present invention relates to a method for treating oil and gas wells. More specifically, the present method serves to clean the bore hole of the well of excess mud prior to cementing and assists in fluid loss control during the cementing process. Because the method cleans the well of excess mud, this allows for improved bonding of the cement to the pipe and to the formation.
2. Description of the Related Art
Previous methods of fluid loss control have been attempted by a one step addition of fluid loss control additive to the cement, which hopefully reduces the ability of the liquid portion of the slurry from rapidly penetrating a permeable zone at the formation face. This creates a critical dependence of the fluid control on the use of a fluid loss control additive that functions at the temperature of the permeable zone. Also, the cement slurry must be designed to complement the requirements of the fluid loss additive for rheology of the fluid portion of the cement slurry. Most fluid loss control additives thicken the cement slurry into which they are mixed. Also, they may retard the hardening of the cement when it reaches its required destination. To deal with such properties, it is common for service companies to have many different fluid loss control additives and to select the “best fit” for the well conditions that are to be encountered.
The current invention is a composition and method that uses a two stage process to allow the use of one cross linking agent composition which can function as a pre wash and “seeding” of the permeable zone of the formation face, and a second or follow up composition which can be the actual cement slurry which contains the reactive polymer composition. This method is not as temperature dependant as most fluid loss additives, but can function from near freezing, i.e. near 32 degrees F., to above 300 degrees F.
A cross linking agent is to be used as a pre wash or as a “spacer” additive or drilling mud additive which is non toxic and environmentally clean. As a chemical wash, the cost is very low compared to complex chemical washes. The cross linking agent can be formulated as a non thickened water base fluid that will allow turbulent flow in the annulus at very low pump rates. The turbulent flow has been observed in laboratory simulations that show the churning action that occurs at the front of the injected wash. The detergent action of the cross linking agent together with the turbulent flow will granulate the excess mud adhering to the pipe and the loose mud at the formation face and carry the material out of the well. During this cleaning action, a “seeding” will take place at the formation face which impregnates the mud cake with cross linking agent chemical. The concentration will vary depending on the permeability at the formation face; the higher concentration going into the more permeable zones.
The cross linking agent can also be used in drilling mud or “spacers” to be available when later injection of polymer takes places. Thus fluid loss control could be obtained without the necessity of the cross linking agent wash, which in certain circumstances might not be desired. There is the possibility that a wash is not always an option if disposal of fluids is a problem.
The polymer can be added to the cement mixture as a dry or liquid fluid loss control additive that requires a cross linking agent wash to be used ahead or to be used when the drilling mud or “spacer” has previously carried the cross linking agent. The polymer is also very low in cost compared to complex fluid loss control additives and requires an order of magnitude less material than conventional additives. This is primarily the result of the permeability block occurring at the formation cement interface where the reaction of the polymer and cross linking agent takes place. If the reaction had taken place in the cement slurry prior to reaching the permeable formation, the slurry would be much thicker and usually would have to contain more water to reduce the viscosity of the reacted chemicals. Concentrating the reaction at the pressure differential interface means that cement slurry doesn't have to carry materials which can cause excessive viscosity or otherwise excessive amount of fluid loss control material. This frees the slurry designer from the excess viscosity of most fluid loss control methods. Testing at the low temperatures has shown fluid loss control with less than 0.1 percent fluid loss control additive in the cement slurry, whereas a one stage fluid loss test using conventional methods and materials requires 1 percent or more.
This two stage method and also the type of chemicals used in the method should be a significant improvement for fluid loss prevention and permeable interface sealing. Such a technique or method could be utilized in any type of permeable situation to minimize leak off of fluid from a well bore, pond, lake, dam, etc.